This invention relates generally to the field of portable computers, and more specifically to a design for laptop and notebook computers wherein the CPU and motherboard is contained in a substantially planar module that mechanically separates away from a display module when the computer is being used.
1. Definitions
The term portable computer includes laptops and notebook computers, and some Personal Digital Assistants. Typically, these computers have a flat-panel display connected to a base by a hinge. The display is shut for transport or storage, and rotated open for use. The base may contain an integral or removable keyboard on the top surface, storage media, batteries, and other components. A portable computer may also feature other user-interface systems, such as a pen-based interface, instead of, or in addition to, a keyboard.
CPU module is defined as a housing containing a substantially planar printed-circuit board or assembly of boards, generally in the same plane, containing at least the central-processing unit (CPU), and possibly additional integrated circuits and other components that may be included on a motherboard.
The flat-panel display module is defined as a flat-panel display, such as an liquid crystal display, in a housing comprised of a bezel and a rear cover in a clam-shell configuration.
2. Background--Description of Prior Art
The rapid growth of the portable computer market demonstrates that computer users prefer the freedom to work in different locations that these computers afford. Increasingly, portables are being purchased by both individuals and large firms as desktop replacement computers. As a result, there is a need for portables that can provide performance comparable to desktop models. Performance is considered to be a combination of the fastest CPUs and support circuitry; the ability to handle a range of media types such as high capacity hard disk drives and CD-ROMs; fast, high resolution video processing; and connectivity functionality provided by networking and other ports. Unfortunately, there is a problem combining all of these components into a single, small enclosure. As the system tends toward thermal equilibrium, the thermal sum of the components raises the temperature above the specified operating temperature of some or all of the components. CPUs in particular have a proportional relationship between processing power and thermal output.
Another performance criteria for portable computers is battery life. Some portables use a fan to cool the hot components. The combination of high performance, high thermal output components, and a fan to cool them, add up to an increase in power drain that reduces battery life.
The size of portable computers is one of the most important performance constraints. Given similar computing performance features, users prefer to purchase the product with the smallest form factor. In fact, many consumers make the purchase decision based on the advertised length, width, and thickness dimensions of the product.
U.S. Pat. No. 5,313,362 to Hatada et al (1994) shows a portable computer with the motherboard located in the base of the unit. This design does not anticipate the new art shown herein for several reasons. First, the location of the motherboard is in the base, not in a mechanically and thermally separate module. All of the heat producing components are located within the base enclosure and are thus thermally coupled. At thermal equilibrium, the temperature in the CPU will be limited by the most thermally sensitive component. Second, FIG. 3 of Hatada et al shows a radiator located in a rear base protrusion. This radiator substantially increases the footprint of the computer and is therefore undesirable in a portable or desktop version where desk space is highly valued. FIG. 10 of Hatada et al shows an auxiliary radiator that is able to rotate away from the flat-panel display so that heat is not transferred to the display. The Hatada et al art relies on convection through vent holes in the main base housing. Thus, the base cannot be effectively sealed from the intrusion of foreign objects, dust, or spilled liquids. Additionally, cooling efficiency is inversely related to thinness since the airflow must have sufficient unrestricted volume to be effective. As the space between the bottom radiator and base housing increases, the unit becomes taller. Furthermore, natural convection off of the bottom side of the primary, high-temperature radiator, is approximately half as efficient as that off the vertical radiator. If vent holes are not used, Hatada et al relies on conduction and convection of heat from external surfaces that the user can touch. The total amount of heat that can be radiated from the computer is limited by the surface touch temperature, shown in Hatada et al FIG. 6 as 55 degrees C. In this case, an increase in heat dissipation can only be realized by an increase in surface area, and thus an increase in the size of the unit.
Both U.S. Pat. No. 4,980,848 to Griffin et al (1990), and U.S. Pat. No. 4,084,213 to Kirchner et al (1978) show a computer having a circuit board with a plurality of heat producing components mounted directly behind the flat-panel display in a single housing. When this display assembly is opened, the motherboard is located in an inclined position. Vents are located at the top and bottom of the assembly housing allowing air to pass between the flat-panel display and the motherboard.
These configurations are not viable for several reasons. First, consumers demand the maximum computing power in the smallest package, thus, laptop and notebook computers are under tight size constraints. As display sizes increase, thickness becomes an important metric; the thinner the computer, the smaller it appears when closed and the easier it is to transport. Thinness is inversely related to thermal efficiency in both the Griffin et al and Kirchner et al art. If the air spaces on either side of the motherboard are minimized, airflow is restricted and there is less convective cooling effect. Moreover, in this configuration with the motherboard in close proximity to the flat-panel display, the uneven, high temperatures on the motherboard will be transferred to the flat-panel display causing irregularities in display contrast that could make the display unreadable. In fact, a fast, hot CPU could push the flat-panel display past its specified operating temperature, rendering it inoperable. Compounding this problem is that the majority of the heat from the CPU and motherboard must be dissipated to the flat-panel display side of the motherboard. This is due to the fact that there is a touch temperature limit on the rear surface of the display/motherboard housing. If a large quantity of heat is allowed to reach this outer surface it could burn a user. The result is that the CPU is limited in its thermal, and thus, it's computational power output.
Alternatively, if the spaces between the motherboard, flat-panel display, and rear cover are increased to make the airflow less restrictive, the overall thickness of the computer increases, making it bulky and inconvenient to carry and stow. Griffin et al describes bosses that support the circuit board being of sufficient length so that the boards are spaced a substantial distance from the LCD, thus the computer is substantially thicker than a conventional design with the CPU and motherboard in the base.
Another problem with Griffin et al and Kirchner et al is that both show vent holes that allow foreign objects or spilled liquids to enter the housing, potentially causing physical damage or shorts on the motherboard. If the vents are made smaller to prevent this, airflow is restricted, thereby decreasing the convective cooling efficiency of the design.
Additionally, the vent holes prevent the housing from being used as a shield element to fully contain electromagnetic interference (EMI). Electronic devices such as portable computers must be shielded so that they emit a small amount of EMI, as prescribed by law. As processor speeds increase, the wavelength of the EMI energy decreases, with the result that more energy will escape through a given size opening in the shield. Many computers use the housing as a shield element. Plastic housings may be coated on the inside surface with a thin layer of electrically conductive material. Griffin et al and Kirchner et al preclude the use of the outer housing as the only EMI shield since EMI will escape from the sizable thermal vent holes.
The Griffin et al and Kirchner et al art also inherently limits the efficiency of heat transfer when a fan is added to the system for forced convection due to the restriction created by the vent holes.